Environmental Engineering Reference
In-Depth Information
5.
Stagnant solution conditions lead to pitting, whereas even a very susceptible
material remains free from pitting in an aggressive medium in flowing condi-
tion.
6.
Stainless steels and aluminum and its alloys are particularly susceptible to
pitting. Carbon steels are more resistant to pitting than stainless steels. Most
failure for stainless steels occurs in neutral-to-acid chloride solutions. Alumi-
num and steels pit in alkaline chloride solutions.
7.
Most pitting is associated with halide ions, with chlorides, bromides, and
hypochlorites being particularly aggressive. Cupric, ferric, and mercuric ha-
lides are extremely aggressive because their cations are cathodically reduced
and sustain the attack.
3.4.2 Evaluation of Pitting Damage
Weight loss methods are inappropriate for pitting damage evaluation because the
negligible loss in weight is not indicative of the seriousness of damage. Extent
of pitting damage can be characterized by pit density, surface size, and depth
with the help of standard rating chart provided in Standard Practice G46-76 of
American Society for Testing and Materials (ASTM) (Fig. 3.6). For example, a
pitted specimen having average spacing (surface density) of 1
10
4
/m
2
, surface
diameter of 8 mm
2
, and pit depth of 3.2 mm will be characterized as A-2, B-3,
C-4. However, for the sake of component life prediction and reliability pit depth,
rather the maximum pit depth, is of significance. Maximum pit depth increases
with time and with sample size. The probability of finding the pit of a certain
depth is higher as the specimen size becomes larger.
Several methods are adopted to measure the pit depth: metallographic, machin-
ing, micrometer depth gauge, and microscopic. The first two are destructive. In
the metallographic method, the sample is sectioned and polished through a se-
lected pit followed by microscopic measurements. The method is tedious and
uncertainty prevails in respect to selection of the deepest pit. Machining out to
a depth where no evidence of pit remains obviously will require samples of regu-
lar shape. In the method using a micrometer, readings between surface and pit
bottoms are compared with a needle probe; in the microscopic method, calibrated
fine focus is used to determine depth difference between surface and pit bottoms.
In both cases, pits must have a large opening for the insertion of the needle probe
or for the light to reach the bottom of the pit. Both fail in proper assessment if
the pit is directionally oriented or undercut.
A special ultrasonic thickness testing has been reported [3] that is capable of
determining the pit depth. In this technique the pit is used as an acoustic lens to
ensure strong front surface and back surface reflection (Fig. 3.7). The ultrasonic
transducer is centered over the pit center by maximizing the front surface signal
arrival time. This is accomplished by using a microcomputer that is also capable
